Natural gas is commonly recovered in areas where natural gas production exceeds demand. In these areas, it is economical to convert the vapor natural gas stream into a liquefied natural gas (LNG) stream. LNG can be stored much more efficiently than vapor natural gas, which makes it economical to transport natural gas to areas distant from the mining region. For transportation purposes, LNG is maintained at cryogenic temperatures so as to retain its liquid state. When LNG reaches its final destination, it is typically heated to return the LNG to a vapor state for transmission through natural gas pipelines to consumers.
LNG consists primarily of saturated hydrocarbon components such as methane, ethane, propane, butane, etc. LNG also may contain trace quantities of nitrogen, carbon dioxide, and oxygen. The stream normally contains more than 50 mole % methane. However, the present invention is not limited to a specified composition of LNG.
LNG often has a higher BTU content or a lower dew point than pipeline specifications will allow. Thus, LNG may require processing before it can be introduced into a natural gas pipeline system. Processing generally requires removal of heavier components from the LNG, which consists of predominantly methane with lesser amounts of heavier hydrocarbons, including ethane, propane, butane, pentane, etc. Prior methods of recovery focus on recovery of these heavier components at the final destination of the LNG product, where one aspect of processing is revaporization of the methane stream.
Generally, pipeline specifications for natural gas require a light stream consisting primarily of methane. Similarly, ethane is generally the most volatile component that can be recovered in any substantial quantity for sale as liquefied petroleum gas (LPG). Thus, processing of LNG generally requires splitting LNG into a methane-rich component and an ethane-rich component. Typically, the methane-rich component will be primarily composed of methane, and the ethane-rich component will be primarily composed of ethane and heavier hydrocarbons.
When LNG is processed at its destination point for purposes of shipping, the methane-rich component is typically recovered as a gas for introduction into natural gas pipeline systems, and the ethane-rich component is typically recovered as a liquid for further processing or sale as LPG. This ethane-rich component, or LPG, may also be referred to as natural gas liquid (NGL).
Various methods for recovering LPG from LNG are known. Marshall (U.S. Pat. No. 2,952,984) teaches the use of a distillation column for separating methane from liquefied natural gas. The method produces a vapor methane product. Markbreiter et al. (U.S. Pat. No. 3,837,172) teach a method for processing LNG but without an external energy input for compression. This method also produces a vapor methane stream. Rambo et al. (U.S. Pat. No. 5,114,451) describe a method for processing LNG focused on recovery of ethane and heavier hydrocarbons and on low capital investment. The method also produces a vapor methane stream.
In contrast, the methane-rich component must be recovered as a liquid when LNG is processed at a location prior to reaching its final shipping destination. Recovery as a liquid allows for efficient transportation to the final destination. The prior art techniques do not provide for removal of C2+ hydrocarbons from a liquefied gas stream and do not take advantage of significant efficiency improvements that are possible where a liquid methane product stream is desired.
LNG is commonly separated to recover heavier hydrocarbons that may be more valuable as liquid products and to bring the LNG stream into compliance with pipeline specifications for component concentrations, dew point, and/or heating value. Therefore, processing generally involves removal of heavier hydrocarbons to decrease specific gravity and to lower heating value or to recover the heavier hydrocarbons as a liquid product for separate sale.